Assuming this isn't a paid position (please correct me if I'm wrong), then you're going to need to really spark an artist's interest in your project - especially as this seems to be intended to be a years-long investment of your time and theirs. Speaking from my point of view as an artist, in order to become involved in collaborating on a project like thus, I would want:
Someone I can work with. You've said what you want from an artist in your post, now it's your turn. You have to present who you are as a writer, a communicator, and a person. After all, we might be working together for a long time, and if we have a consistent failure to communicate or negotiate, I'm going to bail. (This is going to be at least partly personal compatibility.) Expected responsibilities of both artist and writer should also be drawn out here.
A great story, characters, plot, worldbuilding etc. Write it out and make me buy in, make me curious and interested and want to bring it to life!
In a purely practical sense, well... go to forums, groups, social media, etc. and get posting. You're going to have to market yourself and your story to lure in interest. Pull out all the stops and show you're willing to invest as much as you expect from your artist. ^.^